The key sentence in the letter is this, “‘Denialist’ is an ad hominem argument, the meaning of which is defined entirely by the user, intended to discredit the accused without evidence.”
The “anti-denialism” campaign is, to use a word I rarely employ, a literal conspiracy–albeit something of an open one in that it’s openly pushed by Chris Mooney. (Inset.) The purpose is two-fold.
1) Brand those with the “wrong” scientific views not just as “kooks” or “nuts” but as literally pathological. This from a recent article in The New Scientist:
Instigators of denialist movements have more serious psychological problems than most of their followers. ‘They display all the features of paranoid personality disorder [according to one quoted "expert"]‘ “including anger, intolerance of criticism, and what psychiatrists call a grandiose sense of their own importance.” The “expert” goes on to say, “Ultimately, their denialism is a mental health problem. That is why these movements all have the same features, especially the underlying conspiracy theory.
2) Lump those whose ideas you wish to defame with people who truly are whacko. Thus there’s no difference between not accepting the party line on global warming and believing vaccines cause autism or HIV doesn’t cause AIDS.
It is truly insidious and we’re going to be hearing a lot more from these people.